I. Field of the Invention: This invention relates generally to cardiac pacemakers, and more particularly to the design of a cardiac pacemaker in which the stimulation rate is under closed loop control and varies in accordance with the contractility of the heat.
II. Discussion of the Prior Art: In accordance with the teachings of an earlier invention described in application Ser. No. 362,903, filed Mar. 29, 1982, and entitled "BIOMEDICAL METHOD AND APPARATUS FOR CONTROLLING THE ADMINISTRATION OF THERAPY TO A PATIENT IN RESPONSE TO CHANGES IN PHYSIOLOGIC DEMAND" (now U.S. Pat. No. 4,686,987), there is disclosed a system for measuring absolute and relative variations in the impedance of the heart, related to the instantaneous stroke volume, and using that impedance signal to control the admistration of medically-related therapy to the patient. In that application, there is described an implantable lead arrangement and associated circuitry whereby, using impedance plethysmography, the instantaneous impedance measured in a chamber of a beating heart can be sensed and used to vary or adjust the pacing rate of an implantable cardiac pacemaker. A drawback of this earlier invention is that under certain circumstances the normal rate response to changes in stroke volume are reversed. For example, when a patient stands up, his stroke volume decreases which would normally result in a decreased pacing rate. However, an increase in rate is necessary to prevent postural hypotension. Thus, a pacing system based only on stroke volume may not respond optimally in all situations.
In addition to positional sensitivity, the control signal proportional to absolute or relative right ventricular impedance (or stroke volume) obtained using the system of the aforereferenced application is rate dependent. While this allows a fairly sophisticated "closed loop" pacing system to be developed, these systems are necessarily more complex than simple "open loop" systems. Thus, a control parameter which is independent of pacing rate would be attractive in that it would allow the use of simpler, and presumably smaller and cheaper, control circuitry.
Finally, an appropriate independent signal can be used in concert with the impedance stroke volume signal of the invention described in U.S. Pat. No. 4,686,987 to resolve the aforementioned ambiguities as to the desired direction of rate change.
In the Koning et al U.S. Pat. No. 4,566,456, there is described a cardiac pacer incorporating means for sensing right ventricular systolic pressure and then using that pressure value to develop a control signal for adjusting the escape interval of an implanted cardiac pacer. In the introductory portion of that patent, there is briefly described other prior art patents and publications relating to pacemaker designs in which the pacing rate is automatically adjusted to accommodate the patient's level of physical activity. The implementation disclosed in the Koning patent as well as those described in certain of the other patents referenced therein require a fairly complex and, as yet, totally unproven sensor. The pressure transducer of Koning, for example, would necessarily be affected by fibrotic encapsulation which would necessarily significantly alter the manner in which the pacer's escape interval is made to vary with exercise within a relatively short time following implantation.